The present invention relates to steam-generators of the natural-circulation type and in particular refers to those steam-generators which include therein a drier and a steam-separator being mounted in series with a bundle of tubes capable of being removed and replaced as soon as serious damages or defects should appear in it.
Steam-generators for pressurized-water nuclear power stations are known, which are provided with a bundle of about 4000 U-tubes, arranged in such a way that the outer circumference of the bundle is nearly circular and hence could be fitted into the cylindrically shaped steam-generator shell. The tubes in that case are arranged in such a way that one of their end sections fills out a first semicircular region of the boiler tubeplate, whilst the other end section lies in a second semicircular region opposite to the one first mentioned.
The tubes are welded to the boiler tubeplate; the latter is welded in turn at its outer (usually upper) peripheral area to the cylindrical shell of the steam-generator and along its inner (usually lower) peripheral area to the headpiece of the steam-generator, which is so arranged that the flowing heat-carrier medium gets distributed inside the tubes of the bundle of tubes. Inside the steam-generator the aforesaid bundle of tubes is furthermore enclosed in addition in an auxiliary shell which in its upper section (steam-generators have a generally vertical development) carries the steam-separator complex which in general is welded thereon.
This auxiliary shell defines inside the steam-generator two different cavities which are connected together at the end sections. One of these cavities is made cylindrical and contains the bundle of tubes, whilst the other is made annular on the outside and designed to convey the secondary liquid to the bottom of the bundle of tubes. These cavities together with the associate communicating openings are so designed that a chimney draught arises to ensure the return of the fluid evaporating in the steam-generator. Above the steam-separator complex the drier unit is arranged, which in general is welded onto the outer shell of the steam-generator.
Such a steam-generator, when used in the thermonuclear field is generally enclosed in a room defined by massive concrete walls, through which the various pipes pass with the primary and the secondary liquid flowing in and out. In addition, there are also the other generally usual connections for apparatus and instruments.
Although the steam-generator is so designed that during its working life damages and defects should be prevented, it is necessary in practice to provide for such an occurrence. For this purpose various openings are usually incorporated into the outer shell or envelope of the steam-generator, through which inspections and lighter repairs may be carried out, for example, the closing-off of damaged tubes in the tube bundle. But serious defects such as the breakage of a large number of tubes in the tube bundle, require complete substitution of the steam-generator. Such a solution is naturally connected with material cost and the use of time and labour. But above all during this time the plant itself is not available. All this means a heavy cost burden.
In view of the importance of the problem of preventing serious failures or defects in a steam-generator, in particular of nuclear power stations, one might think of solutions which would be directed towards replacing the most sensitive components of the steam-generator, i.e., the tube bundle and hence not the steam-generator as a whole. A first solution among all those solutions hitherto proposed in this respect consists, for example, in the housing of the steam-generator being divided up into three different portions which are connected together mechanically by conventional methods. One could then replace only the portion including the tube bundle and also plan in advance all of the necessary steps such as the cutting of the tubes, the removal of the cladding, the dismantling of the supports and mountings, etc.
Another solution might consist in the steam-generator being divided up into two sections, that is, an upper section with the driers and steam-separators, and a lower section with the tube bundle; this latter in addition might be subdivided into modular elements to be replaced in case of emergency.
But both these attempted solutions show considerable disadvantages which as far as the first mentioned solution is concerned follow from the necessity of having parts of considerable mass and large bulk, to be moved inside the space in which the reactor and the primary system of the power station are accommodated, and having to separate part of the connections to the plant such as mountings, piping, etc. Further disadvantages may be found to derive from the second of the solutions mentioned above, further to the aforesaid steps of movement and conveyance, due to the complicated instrument equipment and the consequent greater probability of defects in the region of the tube bundle.